NHS- accrued annual leave, using it on return from Mat Leave.

I was wanting to shorten my week like you said, but they're only allowing me to take it in a big chunk.
No you don't have to pay mat leave back if you go back on part-time. I think it depends more on the hours that you'll have to do (so if you worked full time, you'd work them back faster if that makes sense). The annual leave after mat leave also counts towards those hours that you're "paying back".
But saying that, make sure they put annual leave on your rota. I got paid today £2k less than I was expecting cause they didn't put annual leave on my rota and I'm hoping I can argue that they pay it before next pay in Jan 🙃

I came back part time and did not have to pay my maternity pay back. As long as you return to work and work for 3 months you do not have to pay maternity pay back. As of annual leave I'm not sure I used all that I accrued to delay me coming back and essentially took another month off.

My work is allowing me to do either

Our maternity leave policy says if coming back from maternity leave part time need to take the majority of accrued annual leave before returning back to work. It will depend on the discretion of your manager though.

My work said accrued annual leave needs to be taken as a block following MAT leave you can't go back and work every other week for example.
You can however book your annual leave how you like given there is still availability.
I managed to book off every mon and Fri from start of May until mid July so I'll only work 1 weekend day per week until the free nursery hours kick in

I think it depends on your trust. I’m returning to work in January and I have some full weeks off and other o have a day or two of annual leave which shortens my working week. It’s probably best to either get in touch with your manager or HR